The main difference is the 6 is a square and the 7 is a half hearted rectangle.

This is followed by...
The 6 has the convenience of a collapsing lens mount. This makes the camera thinner than a Pro 35mm.

The 6 has 3 lenses that have been tested to be as sharp as anything out there (50mm, 75mm and 150mm).

The 7 has a better meter, larger lens selection (43mm, 65mm, 80mm, 150mm and 210mm), minor design improvements in the curtin set/release, pc socket and the ability to do multiple exposures (7II).

The 6 has one major draw back. The film advance gears have a tendency to break and are nolonger fixable by Mamiya.